Iron/lithium--promoted catalysts for the production of maleic anhydride

ABSTRACT

Catalysts useful for the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, with molecular oxygen or an oxygen-containing gas in the vapor phase to produce maleic anhydride are provided which comprise phosphorus, vanadium, and oxygen and a promoter component containing each of iron and lithium.

FIELD OF THE INVENTION

This invention relates to phosphorus-vanadium-oxygen oxidationcatalysts. More particularly, this invention relates toiron/lithium-promoted phosphorus-vanadium-oxygen catalysts. Suchcatalysts are useful for the partial oxidation of nonaromatichydrocarbons in the vapor phase with molecular oxygen or a molecularoxygen-containing gas to produce maleic anhydride in excellent yields.

Maleic anhydride is of significant commercial interest throughout theworld. It is used alone or in combination with other acids in themanufacture of alkyd and polyester resins. It is also a versatileintermediate for chemical synthesis. Significant quantities of maleicanhydride are produced each year to satisfy these varied needs.

DESCRIPTION OF THE PRIOR ART

Numerous catalysts containing phosphorus, vanadium, and oxygen(sometimes referred to as mixed oxides of phosphorus and vanadium) aredisclosed in the prior art as being useful for the conversion of variousorganic feedstocks to maleic anhydride, and further that such catalystswherein the valence of the vanadium is below +5, usually between about+3.8 and +4.8, are particularly well suited for the production of maleicanhydride from saturated hydrocarbons having at least four carbon atomsin a straight chain. In many instances, these catalysts also containadded promoter elements or components which are considered to exist inthe catalyst as oxides. Common organic feedstocks include nonaromatichydrocarbons such as n-butane, 1- and 2- butenes, 1,3-butadiene, ormixtures thereof.

Procedures for the preparation of catalysts containing the mixed oxidesof phosphorus and vanadium and promoter components are also disclosedand taught by the prior art. Many of such procedures teach that it ispreferable to reduce the vanadium in solution to the tetravalent state.For example, these catalysts can be prepared by contactingphosphorus-containing compounds, vanadium-containing compounds, andpromoter component-containing compounds under conditions sufficient toproduce the tetravalent vanadium and to form the promotercomponent-containing catalysts precursor. The catalyst precursor isthereafter recovered, dried, and calcined to produce the activecatalyst.

U.S. Pat. No. 4,312,787 describes catalysts which comprise an inertsupport and a catalytically active mixed oxide material coating on theouter surface of the support in an amount greater than 50% to about 80%by weight of the combined support and oxide material. Such coating maybe composed of oxides of phosphorus and vanadium or, alternatively, ofoxides of phosphorus, vanadium, and uranium. Catalysts within the scopeof the claims of the patent are reported to produce maleic anhydridefrom n-butane in yields ranging from 53% to 62.5%, with selectivitiesranging from 57.4% to 67.9%.

In U.S. Pat. No. 4,251,390, a zinc-promoted phosphorus-vanadium-oxygencatalyst is disclosed and claimed. The catalyst is prepared by reducingpentavalent vanadium in a substantially anhydrous organic medium to alower valence state and digesting the reduced vanadium in the presenceof a zinc promoter compound. The resulting catalyst is activated bybringing the catalyst to operating temperatures for the oxidation ofn-butane to maleic anhydride at a rate of 5° C. to 10° C. per hour inthe presence of a butane-in-air mixture.

U.S. Pat. No. 4,018,709 discloses a process for the vapor phaseoxidation of C₄ n-hydrocarbons using catalysts containing vanadium,phosphorus, uranium, or tungsten or a mixture of elements from zinc,chromium, uranium, tungsten, cadmium, nickel, boron, and silicon. In apreferred embodiment, the catalyst also contains an alkali metal or analkaline earth metal, especially lithium, sodium, magnesium, or bariumas active components. Typically, such catalysts are prepared inconcentrated (37%) hydrochloric acid.

U.S. Pat. No. 4,002,650 discloses a process for the oxidation ofn-butane using a catalyst of the formula

    V.sub.0.5-3 P.sub.0.5-3 U.sub.0.1-0.5 O.sub.x

wherein x is a number taken to satisfy the valence requirements of theother elements present. In a preferred preparative procedure, a mixtureof vanadium pentoxide, concentrated hydrochloric acid, and uranylacetate is heated under reflux. To this refluxing mixture is added 85%phosphoric acid. The resulting mixture is evaporated at atmosphericpressure and dried at 110° C., ground and screened to a suitableparticle size, and activated by heating in an air flow at 482° C. forsixteen hours.

In U.S. Pat. No. 3,980,585, a process is disclosed for the preparationof maleic anhydride from normal C₄ hydrocarbons in the presence of acatalyst containing vanadium, phosphorus, copper, oxygen, tellurium or amixture of tellurium and hafnium or uranium or a catalyst containingvanadium, phosphorus, copper, and at least one element selected from thegroup of tellurium, zirconium, nickel, cerium, tungsten, palladium,silver, manganese, chromium, zinc, molybdenum, rhenium, samarium,lanthanum, hafnium, tantalum, thorium, cobalt, uranium, and tin,optionally (and preferably) with an element from Groups 1a (alkalimetals) or 2a (alkaline earth metals).

U.S. Pat. No. 3,888,866 discloses a process for the oxidation ofn-butane by contacting the n-butane at a temperature from about 300° C.to about 600° C. with a phosphorus-vanadium-oxygen catalyst having aphosphorus/vanadium atom ratio of 0.5-2, promoted or modified withchromium, iron, hafnium, zirconium, lanthanum, and cerium, the promotermetal/vanadium atom ratio between about 0.0025 and about 1. Thecatalysts are prepared by refluxing a reaction mixture of vanadiumoxide, phosphoric acid, a hydrogen halide (usually hydrochloric acid),and a specified promoter metal-containing compound. The resultingcatalyst precursors are recovered, dried, formed into structures, andcalcined to produce the active catalysts.

U.S. Pat. No. 3,862,146 discloses a process for the oxidation ofn-butane to maleic anhydride in the presence of aphosphorus-vanadium-oxygen catalyst complex, promoted or activated witha zinc, bismuth, copper, or lithium activator. The phosphorus/vanadiumand activator/vanadium atom ratios are from about 0.5-5 and from about0.05-0.5, respectively.

U.S. Pat. No. 3,856,824 discloses a process for the production of maleicanhydride by oxidation of saturated aliphatic hydrocarbons in thepresence of a catalyst comprising phosphorus, vanadium, iron, oxygen andan added modifier comprising chromium combined with at least one elementselected from the group consisting of nickel, boron, silver, cadmium,and barium.

European Patent Application No. 98,039 discloses a process for thepreparation of phosphorus-vanadium mixed oxide catalyst, optionallycontaining an added promoter element selected from the group consistingof Group 1a (alkali metals), Group 2a (alkaline earth metals), titanium,chromium, tungsten, niobium, tantalum, manganese, thorium, uranium,cobalt, molybdenum, iron, zinc, hafnium, zirconium, nickel, copper,arsenic, antimony, tellurium, bismuth, tin, germanium, cadmium, andlanthanides, and mixtures thereof. The catalyst, which exhibit aphosphorus/vanadium atom ratio from 0.8 to 1.3 and a promoter/vanadiumatom ratio from 0.01 to 0.5, are prepared in an organic liquid reactionmedium capable of reducing the vanadium to a valence state ofapproximately +4 to form a non-solubilized catalyst precursor,contacting the non-solubilized catalyst precursor containing organicliquid with water to form a two-phase system having an upper organicliquid phase and a lower non-solubilized catalyst precursor-containingaqueous phase, drying the catalyst precursor, and calcining. Thecatalysts so obtained reportedly are useful in the production of maleicanhydride from normal C₄ hydrocarbons.

Although these prior art catalysts are effective to provide the desiredproduct, maleic anhydride, the commercial utility of a catalyst systemis highly dependent upon the cost of the system, the conversion of thereactant(s), and the yield of the desired product. In many instances, areduction in the cost of a catalyst system on the order of a few centsper kilogram or pound, a small percent increase in the yield of thedesired product, relative to the amount of catalyst required, representsa tremendous commercial economical saving and advantage. Accordingly,research efforts are continually being made to define new or improvedcatalyst systems and methods and processes of making new and oldcatalyst systems to reduce the cost and/or upgrade the activity andselectivity of such catalyst systems in such processes. The discovery ofthe catalysts of the instant invention, therefore, is believed to be adecided advance in the catalyst art.

SUMMARY OF THE INVENTION

It is an object of this invention to provide improved catalystscomprising phosphorus, vanadium, and oxygen and a promoter componentcontaining each of iron and lithium useful for the oxidation ofnonaromatic hydrocarbons to produce maleic anhydride.

Another object of this invention is to provide improved catalystscomprising phosphorous, vanadium, and oxygen and a promoter componentcontaining each of iron or lithium useful for the production of maleicanhydride in excellent yields.

To achieve these and other objects, together with the advantagesthereof, which will become apparent from the accompanying descriptionand claims, catalysts are provided which comprise phosphorus, vanadium,and oxygen and a promoter component containing each of iron and lithium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with this invention, catalysts are provided which areuseful for the partial oxidation of nonaromatic hydrocarbons having atleast four carbon atoms in a straight chain with molecular oxygen or amolecular oxygen-containing gas in the vapor phase to maleic anhydride.These catalysts, which comprise phosphorus, vanadium, and oxygen and apromoter component containing each of iron and lithium, exhibitexcellent selectivities to, and yields of, maleic anhydride.

The catalysts of the instant invention have a phosphorus-to-vanadium(phosphorus/vanadium or P/V) atom ratio from about 0.50 to about 2.00,with a P/V atom ratio of about 0.95 to about 1.20 being preferred. Thetotal atom ratio of iron and lithium to vanadium[(iron+lithium)/vanadium or (Fe+Li)/V] advantageously is in the rangefrom about 0.0025 to about 0.0080, with the proviso that the Fe/V atomratio is in the range from about 0.0010 to about 0.0040, preferablyabout 0.0015 to about 0.0035, and the Li/V atom ratio is from about0.0015 to about 0.0040, preferably from about 0.0025 to about 0.0035,most preferably about 0.0030, and with the further proviso that theFe/Li atom ratio is from about 0.30 to about 1.30.

For purposes of this invention, the term "yield" means the ratio of themoles of maleic anhydride obtained to the moles of hydrocarbon feedstockintroduced into the reactor multiplied by 100, the term expressed asmole percent. The term "selectivity" means the ratio of moles of maleicanhydride obtained to the moles of hydrocarbon feedstock reacted orconverted multiplied by 100, the term expressed as mole percent. Theterm "conversion" means the ratio of the moles of hydrocarbon feedstockreacted to the moles of hydrocarbon introduced into the reactormultiplied by 100, the term expressed as mole percent. The term "spacevelocity" or "gas hourly space velocity" or "GHSV" means the hourlyvolume of gaseous feed expressed in cubic centimeters (cc) at 20° C. andatmospheric pressure, divided by the catalyst bulk volume, expressed incubic centimeters, the term expressed as cc/cc/hour or hr⁻¹.

Component source materials suitable for use in the instant invention arethose which yield the unique catalysts of the instant invention.Representatives vanadium-containing compounds useful as a source ofvanadium in the catalysts of the instant invention are vanadium oxides,such as vanadium tetroxide and vanadium pentoxide; vanadium oxyhalides,such as vanadyl dichloride, vanadyl trichloride, vanadyl dibromide, andvanadyl tribromide; vanadium-containing acids, such as metavanadic acidand pyrovanadic acid; and vanadium salts, both organic and inorganic,such as ammonium metavanadate, vanadium oxysulfate (vanadyl sulfate),vanadyl formate, vanadyl acetoacetonate, vanadyl oxalate, vanadylalkoxides, and mixtures thereof. Among these compounds, vanadiumpentoxide is preferred.

The phosphorus-containing compounds useful as a source of phosphorus inthe catalysts of the instant invention are those well known to the art.Suitable phosphorous-containing compounds include phosphoric acid, suchas metaphosphoric acid, orthophosphoric acid, triphosphoric acid, andpyrophosphoric acid; phosphorus oxides, such as phosphorus pentoxide;phosphorus halides and oxyhalides, such as phosphorus oxyiodide,phosphorus pentachloride, and phosphorus oxybromide; phosphorus salts,such as mono-, di-, and triammonium phosphates; and organophosphoruscompounds, such as ethyl phosphate and methyl phosphate and mixturesthereof. Of these phosphorus-containing compounds, the phosphoric acids,such as orthophosphoric acid and pyrophosphoric acid and mixturesthereof are preferred. More specifically, phosphoric acid is employed assubstantially anhydrous phosphoric acid, for example, orthophosphoricacid. Polyphosphoric acid is another preferred type of anhydrousphosphoric acid. This latter acid is commercially available as a mixtureof orthophosphoric acid with pyrophosphoric (diphosphoric),triphosphoric, and higher acids, and is sold on the basis of itscalculated content of H₃ PO₄, as, for example 115%. Superphosphoric acidis a similar mixture sold at 105% H₃ PO₄. Such acids (having calculatedH₃ PO₄ concentrations greater than 100% revert primarily toorthophosphoric acid upon dilution with water.

In addition to phosphorus, vanadium, and oxygen, the catalysts of theinstant invention, as previously noted, also comprise a promotercomponent containing each of iron and lithium. Such promoter componentsare readily introduced into the catalysts during the formation of thecatalyst precursor (as discussed herein below) by adding the promotercomponent to the reaction solution as separate compounds together withthe vanadium-containing compound or separately introducing suchcompounds into the reaction solution. The promoter component-containingcompounds, however, should be at least partially soluble in the reactionmedium (alcohol medium and the added anhydrous hydrogen chloride).

As a source of iron for the iron promoter component, variousiron-containing compounds, both ferric and ferrous, may be employed.Suitable iron-containing compounds include iron halides, phosphates,oxides, carbonates, sulfates, nitrates, acetates, oxalates, citrates,and the like. Metallic iron also may be employed, and, in general, isthe iron source material of choice.

The lithium-containing compounds useful as a source material for thelithium promoter component are not narrowly critical. Suitablelithium-containing compounds include lithium halides, phosphate, oxide,hydroxide, carbonate, sulfate, nitrate, acetate, oxalate, citrate, andthe like. Among these compounds, lithium chloride (a lithium halide) isgenerally preferred.

The catalysts of the instant invention, broadly described, are preparedby contacting at least one of each of a vanadium-containing compound, aphosphorus-containing compound, and a promoter component as at least oneof each of an iron-containing compound (including metallic iron) and alithium-containing compound in an alcohol medium (as describedhereinbelow) in a manner and under conditions capable of reducing thevanadium (if required) to a desired valence state (less than +5) in thepresence of anhydrous hydrogen chloride in an amount sufficient todissolve the vanadium-containing compound in the alcohol medium and toreact the phosphorus-containing compound with the reducedvanadium-containing compound and the promoter component to form catalystprecursors, recovering the catalyst precursors, forming the catalystprecursors into structures (if structures are desired), and calciningthe catalyst precursors to form the catalysts.

The contacting of the vanadium-containing compound, thephosphorus-containing compound and the promoter component may beaccomplished in any convenient manner. In one embodiment, thephosphorus-containing compound may be introduced into a suspension(solution) of the vanadium-containing compound/promotercomponent/alcohol medium mixture, either prior to or subsequent to theaddition of the anhydrous hydrogen chloride gas, in the form of asolution or suspension in the alcohol medium, or, when thephosphorus-containing compound is in liquid form, such as 100%phosphoric acid, it may be added alone. Alternatively, thevanadium-containing compound, the phosphorus-containing compound, andthe promoter component can be introduced simultaneously into the alcoholmedium, followed by contacting the mixture with the anhydrous hydrogenchloride gas. In yet another mode, the vanadium-containing compound andthe promoter component are introduced into a solution or dispersion ofthe phosphorus-containing compound in the alcohol medium. In a preferredembodiment, however, the vanadium-containing compound and the promotercomponent are introduced into a solution of the phosphorus-containingcompound in the alcohol medium and the mixture contacted with anhydroushydrogen chloride gas.

The alcohols suitable for use as the alcohol medium in the preparativeprocess for the catalysts of the instant invention must be capable offunctioning as a solvent and/or suspending agent for thevanadium-containing compound and the promoter component-containingcompounds, as a solvent and/or diluent for the phosphorus-containingcompound, and where needed, a mild reducing agent for thevanadium-containing compound and preferably, as a suspending agent forthe catalyst precursors. Thus, it is preferred that the alcohol is not asolvent for the catalyst precursors. In those instances wherein thecatalyst precursor is soluble in the alcohol medium, however,precipitation should be readily induced by removal of a portion of thealcohol. Suitable alcohols include primary and secondary alcohols, suchas methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-1-propanol(isobutyl alcohol), 3-methyl-2-butanol, 2,2-dimethyl-1-propanol,1,2-ethanediol (ethylene glycol). Of these alcohols, isobutyl alcohol ispreferred because of its ready availability, its relatively low cost,and its convenient boiling point (108° C.).

When a pentavalent vanadium-containing compound, such as the preferredvanadium pentoxide, V₂ O₅, is employed as the vanadium source material,it must be reduced (at least in part) to the tetravalent state. Thedesired reduction is readily accomplished by contacting the pentavalentvanadium with the alcohol medium (and the anhydrous hydrogen chloride)either in the presence or absence of the phosphorus-containing compoundand the promoter component, and the resulting mixture heated to atemperature sufficient to achieve the appropriate average vanadiumvalence state. As is well known to those skilled in the art, anhydroushydrogen chloride can serve to dissolve vanadium-containing compounds inan alcohol medium and thereby enhance the rate of pentavalent vanadiumreduction. Preferably, the pentavalent vanadium, as previously noted, isonly partially reduced by heating it in the alcohol medium until thedesired valence state of less than +5 is achieved, most preferably anaverage valence state between about +3.9 to about +4.6 or simply about+3.9 to about 4.6. In general, suitable reduction is indicated to havebeen achieved when the color of the reaction mixture (solution) turnsblue, such color being indicative of a vanadium valence of between about3.9 to about 4.6.

The vanadium reduction temperature will depend on the reducing strengthof the alcohol medium selected and can vary widely. Accordingly, whileany temperature effective to reduce the vanadium-containing compound issuitable, such effective temperatures typically will vary from about 30°C. to about 300° C., preferably from about 60° C. to about 200° C., andmost preferably from about 80° C. to about 150° C. Preferably, thealcohol medium selected will boil at about the selected temperature sothe reaction can be conducted by refluxing. Thus, when isobutyl alcoholis used as the alcohol medium, simple refluxing at about 108° C.[1.013×10² kPa-G (1 atm)] for a period of from about five hours to abouteight hours will suffice. The reaction mixture preferably is maintainedin the substantially anhydrous state by removing any water formed insitu by azeotropic distillation or other suitable means. By"substantially anhydrous" as used herein is meant typically less thanabout 10%, preferably less than about 5%, and most preferably less thanabout 1%, by weight water, based on the weight of the alcohol medium inthe reaction mixture. As previously noted, anhydrous hydrogen chlorideserves to dissolve the vanadium-containing compound to create ahomogeneous solution of the vanadium containing compound in the alcoholmedium and thereby enhance the rate of reduction of the pentavalentvanadium.

The alcohol medium is employed in amounts effective to achieve theappropriate vanadium reduction, where needed, to provide uniform heatingof the vanadium-containing compound, and preferably to provide asolution which can be conveniently refluxed at the selected reductiontemperature. Thus, while any effective amount of alcohol can beemployed, such effective amounts typically will constitute from about50% to about 90% by weight, based on the combined weight of the alcoholmedium the vanadium-containing compound and, if present, the promotercomponent.

When the reduction of the vanadium-containing compound to a valencestate of less than +5 is carried out in the absence of either or both ofthe phosphorus-containing compound and/or the promoter component, theentire solution of alcohol medium and the reduced vanadium-containingcompound is cooled to a temperature between about 20° C. and about 50°C., and combined with a solution of the phosphorus-containing compounddissolved in a similar, preferably the same, alcohol medium and, ifnecessary, the promoter component to form a reaction mixture. Thereaction mixture is then heated, preferably refluxed, to reduce and/orreact the vanadium-containing compound with the phosphorus-containingcompound (and the promoter component) at temperatures of typically fromabout 30° C. to about 300° C., preferably from about 60° C. to about200° C., and most preferably from about 80° C. to about 150° C., for aperiod of typically from about one hour to about 50 hours, preferablyfrom about 10 hours to about 35 hours, and most preferably from about 15hours to about 25 hours to form the catalyst precursor. The abovereaction, as previously noted for the vanadium-reduction reaction,preferably is conducted to maintain the reaction mixture in thesubstantially anhydrous state, preferably by azeotropic distillation toremove any water formed in situ.

In those instances where the vanadium-containing compound alreadypossesses an average vanadium valence of between about 3.9 and about4.6, the separate vanadium-reduction step can be eliminated and thevanadium-containing compound reacted directly with thephosphorus-containing compound (and the promoter component) in thealcohol medium as previously described.

The reaction pressure for the catalyst precursor-forming reaction is notcritical and can be subatmospheric, atmospheric, or superatmospheric,provided the reactants and alcohol medium do not volatilize to such anextent that the composition of the reaction mixture is alteredsubstantially from the description provided herein. Atmospheric pressureis preferred.

Advantageously, the catalyst precursor-forming reaction is conductedunder sufficient agitation to assure uniform reacting, and interactionbetween the reactants, during reaction. This can be achieved byconventional high speed agitation equipment capable of achieving a highdegree of mixing.

Upon completion of the reaction, it is necessary to recover theresulting catalyst precursor from the substantially homogeneous reactionmixture. In general, this may be achieved by removing a portion of thealcohol medium to induce precipitation of the catalyst precursor. Thereaction mixture may then be cooled to between 20° C. and 50° C. and thecatalyst precursor separated from the alcohol medium. This separationcan be accomplished by a variety of conventional techniques well knownto those skilled in the art, including filtration, centrifugation anddecantation of the supernatent liquid alcohol medium from the solidcatalyst precursor, and evaporating the alcohol medium to form a cake orpaste of the catalyst precursor.

The recovered catalyst precursor is then typically subjected toconditions sufficient to remove most of the remaining alcohol. This canbe achieved by drying, preferably continuous drying, to evaporate suchalcohol. Before final drying is conducted, if desired, the recoveredcatalyst precursor can be washed in the alcohol (of the alcohol medium)one or more times to remove any residual unreacted phosphorus-containingcompound and/or any other alcohol-soluble species occluded in thecatalyst precursor, followed by a repetition of the catalyst precursorrecovery procedures previously described.

Drying can be achieved by exposing the catalyst precursor to air at roomtemperature for a period of from about one hour to about 100 hours or byplacing it in a forced hot air oven maintained at a temperature of lessthan about 180° C., typically between about 100° C. and about 150° C.for about one hour to about 10 hours. Alternatively, the catalystprecursor can be air dried at room temperature for between about onehour and about 48 hours and then placed in the forced hot air oven.Drying of the catalyst precursor preferably should be conducted attemperatures below those at which crystal phase transitions occur anduntil a level of nearly constant weight is achieved. Drying underreduced pressure at room or elevated temperature, as previouslydescribed, also can be employed as a suitable alternative.

After the catalyst precursor has been recovered and dried, it is thenformed into structures, if structures are desired, suitable for use in amaleic anhydride reactor, although nonstructured, powder can beemployed. Techniques for forming appropriate structures from thecatalyst precursors for use in a fixed bed, heat exchanger type reactoror in a fluidized bed reactor are well known to those skilled in theart. For example, the catalyst precursors can be structured inunsupported form for use in a fixed bed, heat exchanger type reactor byprilling or tableting, extruding, sizing, and the like. Suitable bindingand/or lubricating agents for pelleting or tableting include Sterotex®,starch, calcium stearates, stearic acid, and graphite. Extrusion of thecatalyst precursor can be achieved by forming a wet paste which does notslump and extruding the paste. Similarly, the catalyst precursors can becomminuted for use in a fluidized bed reactor.

The catalyst precursors also can be supported on support materials orcarriers for use in either fixed or fluidized bed operations.Nonlimiting representative carriers include alumina, silica,silica-gel,silicon carbide, ceramic donuts, magnesia, titania, andtitania-silica.

In a preferred embodiment, the catalyst precursor, whether structured,nonstructured, supported, nonsupported, or any combination thereof, isroasted at a temperature from about 200° C. to about 290° C., preferablyfrom about 250° C. to about 275° C., for a suitable period of time,usually at least two hours, preferably from about four hours to abouteight hours, to remove residual traces of organic materials. In a mostpreferred embodiment, the catalyst precursors are roasted by heating ina nitrogen-purged furnace to about 260° C. over a one-hour period,maintaining this temperature over an additional six-hour period, andpurging the roasting furnace with dry air at the (beginning of the)fourth hour of the temperature maintenance or hold period, theembodiment conveniently designated as 1(260)6 roasting.

The catalyst precursors, prior to use, must be calcined/activated(hereinafter conveniently referred to as calcined or cognate words, suchas calcine and calcination) in order to convert the catalyst precursorinto the active catalyst. This may be accomplished by heating thecatalyst precursor in a selected atmosphere at a selected elevatedtemperature either in a separate step or, preferably, in situ in thereactor in which the catalyst will be used for the production of maleicanhydride. During such calcination, it is desirable, although notessential, to maintain a steady flow of the calcination atmosphere overthe catalyst precursor surface. Suitable space velocities for theatmosphere typically range from about 50 hr⁻¹ to about 150 hr⁻¹, usuallyabout 100 hr⁻¹.

In a preferred embodiment, the catalyst precursor is charged to themaleic anhydride reactor and heated in a dry air atmosphere flowing atthe previously noted space velocity to a temperature from about 100° C.to about 290° C., preferably from about 250° C. to about 290° C., for asuitable period of time, usually about two hours. Water is thenoptionally added to the flowing dry air stream in an amount sufficientto provide a water concentration up to about 10% by volume. Normally, awater concentration of about 1.5 volume percent to about two volumepercent is sufficient. Thereafter, the temperature is increased to avalue from about 300° C. to about 400° C., usually from about 350° C. toabout 400° C., at a maximum rate of about 10° C. per hour, normally fromabout 1° C. to about 3° C. per hour, and any gaseous hydrocarbondescribed hereinafter as suitable for partial oxidation to maleicanhydride, preferably n-butane, is added to the flowing air stream incontact with the catalyst in an amount sufficient to provide ahydrocarbon concentration from about 0.5 mole percent to about 1.5 molepercent, preferably about 0.6 mole percent. The hydrocarbon isintroduced at a temperature less than the phase transformationinitiation temperature (normally from about 300° C. to about 315° C.). Ahydrocarbon introduction temperature from about 275° C. to about 290° C.is preferred. The calcination temperature typically is maintained over aperiod ranging from about 0.5 hour to about 24 hours, preferably fromabout one hour to about six hours.

The catalysts of the instant invention, as previously noted, exhibit aP/V atom ratio from about 0.50 to about 2.00, with a P/V atom ratio ofabout 0.95 to about 1.20 being preferred. The catalysts also exhibit atotal (Fe+Li)/V atom ratio from about 0.0025 to about 0.0080, with theproviso that the Fe/V atom ratio is from about 0.0010 to about 0.0040,preferably from about 0.0015 to about 0.0035 and the Li/V atom ratio isfrom about 0.0015 to about 0.0040, preferably from about 0.0025 to about0.0035, most preferably about 0.0030, and with the further proviso thatthe Fe/Li atom ratio is from about 0.30 to about 1.30.

The catalysts of the instant invention can be used (in a suitablereactor) to convert nonaromatic hydrocarbons to maleic anhydride. Amixture of hydrocarbon and a molecular oxygen-containing gas (includingmolecular oxygen), such as air, can be contacted with the catalysts attemperatures between about 300° C. and 600° C. at concentrations of fromabout one mole percent to about 10 mole percent hydrocarbon at a gashourly space velocity (GHSV), or simply space velocity, up to about 4000hr⁻¹ to produce maleic anhydride. However, the initial yield of maleicanhydride may be low; and if this is the case, the catalyst, as willoccur to those skilled in the art, can be "conditioned" by contactingthe catalyst with low concentrations of hydrocarbon and maleicoxygen-containing gas at low space velocities for a period of timebefore production operations begin.

The reaction to convert nonaromatic hydrocarbons to maleic anhydriderequires only contacting the hydrocarbons admixed with a molecularoxygen-containing gas (including molecular oxygen), such as air ormolecular oxygen-enriched air, with the catalyst at elevatedtemperatures. In addition to the hydrocarbon and molecular oxygen, othergases, such as nitrogen or steam, may be present or added to thereactant feed stream. Typically, the hydrocarbon is admixed with themolecular oxygen-containing gas, preferably air, at a concentration ofabout one mole percent to about 10 mole percent hydrocarbon andcontacted with the catalysts at a space velocity of about 100 hr⁻¹ toabout 4000 hr⁻¹ at a temperature between about 300° C. and about 600°C., preferably from about 1000 hr⁻¹ to about 3000 hr⁻¹ and from about325° C. to about 425° C., to provide an excellent yield of, andselectivity to, maleic anhydride.

The catalysts of the instant invention are useful in a variety ofreactors to convert nonaromatic hydrocarbon to maleic anhydride. Thecatalysts may be used in a fixed-bed reactor using any of the structurespreviously described, such as, for example, tablets or pellets, or in afluid-bed reactor using catalysts preferably having a particle size ofless than 300 microns (μm). Details of the operation of such reactorsare well known to those skilled in the art.

The catalysts of the instant invention are particularly useful in fixedbed (tube), heat exchanger type reactors. The tubes of such reactors canvary in diameter from about 0.635 cm (0.25 in.) to about 3.81 cm (1.50in.) and the length can vary from about 15.24 cm (6 in.) to about 304.80cm (10 ft) or more. It is desirable to have the surfaces of the reactorsat relatively constant temperatures, and some medium to conduct heatfrom the reactors is necessary to aid temperature control. Nonlimitingexamples of such media include Woods metal, molten sulfur, mercury,molten lead, and eutectic salt baths. A metal block reactor whereby themetal surrounding the tube acts as a temperature regulating body alsocan be used. The reactor or reactors can be constructed of iron,stainless steel, carbon steel, nickel, glass, such as Vycor, and thelike.

Pressure is not critical in the reaction to convert nonaromatichydrocarbons to maleic anhydride. The reaction may be conducted atatmospheric, superatmospheric, or subatmospheric pressure. It generallywill be preferred, however, for practical reasons, to conduct thereaction at or near atmospheric pressure. Typically, pressures fromabout 1.013×10² kilopascals-gauge (kPa-G, 14.7 psig, 1 atm) to about1.38×10² kPa-G (20.0 psig) may be conveniently employed.

Maleic anhydride produced by using the catalysts of the instantinvention can be recovered by any means well known to those skilled inthe art. For example, maleic anhydride can be recovered by directcondensation or by absorption in suitable media with subsequentseparation and purification of the anhydride.

A large number of nonaromatic hydrocarbons having from four to 10 carbonatoms can be converted to maleic anhydride using the catalysts of theinstant invention. It is only necessary that the hydrocarbon contain notless than four carbon atoms in a straight chain. As an example, thesaturated hydrocarbon n-butane is satisfactory, but isobutane(2-methylpropane) is not satisfactory for conversion to maleic anhydridealthough its presence is not harmful. In addition to n-butane, othersuitable saturated hydrocarbons include the pentanes, the hexanes, theheptanes, the octanes, the nonanes, the decanes, and mixtures of any ofthese, with or without n-butane so long as a hydrocarbon chain having atleast four carbon atoms in a straight chain is present in the saturatedhydrocarbon molecule.

Unsaturated hydrocarbons are also suitable for conversion to maleicanhydride using the catalysts of the instant invention. Suitableunsaturated hydrocarbons include the butenes, (1-butene and 2-butene),1,3 butadiene, the pentenes, the hexenes, the heptenes, the octenes, thenonenes, the decenes, and mixtures of any of these, with or without thebutenes, again, so long as the requisite hydrocarbon chain having atleast four carbon atoms in a straight chain is present in the molecule.

Cyclic compounds such as cyclopentane and cyclopentene are alsosatisfactory feed materials for conversion to maleic anhydride.

Of the aforementioned feedstocks, n-butane is the preferred saturatedhydrocarbon and the butenes are the preferred unsaturated hydrocarbons,with n-butane being most preferred of all feedstocks.

It will be noted that the aforementioned feedstocks need not necessarilybe pure substances, but can be technical grade hydrocarbons.

The principal product from the oxidation of the aforementioned suitablefeed materials is maleic anhydride, although small amounts of citraconicanhydride (methylmaleic anhydride) may also be produced when thefeedstock is a hydrocarbon containing more than four carbon atoms.

The following specific examples illustrating the best currently-knownmethod of practicing this invention are described in detail in order tofacilitate a clear understanding of the invention. It should beunderstood, however, that the detailed expositions of the application ofthe invention, while indicating preferred embodiments, are given by wayof illustration only and are not to be construed as limiting theinvention since various changes and modifications within the spirit ofthe invention will become apparent to those skilled in the art from thisdetailed description.

EXAMPLE 1

(a) Orthophosphoric Acid, 100%

A 3-liter, 4-neck, round bottom flask, equipped with a thermometer and astainless steel paddle stirrer, was charged with 901.8 g (7.87 moles) of85.5% orthophosphoric (phosphoric) acid (H₃ PO₄). Stirring was commencedand 343.4 g (2.42 moles) of phosphorus pentoxide (P₂ O₅) was added tothe phosphoric acid, causing an exothermic reaction and an increase intemperature as high as 150° C. as the P₂ O₅ dissolved. The resultantsolution was stirred for 20 minutes at the elevated temperatures andthereafter cooled to ambient temperatures (approximately 25° C.).

(b) Catalysts

A 12-liter, 4-neck, round bottom flask equipped with a thermometer,coarse-frit gas dispersion tube, a paddle stirrer, and a water-cooledDean Stark trap fitted with a Friedrich's condenser was charged with 8.31 of isobutyl alcohol. Stirring was commenced and the isobutyl alcoholwas cooled to 10°-15° C. To the cooled isobutyl alcohol was added over a12-minute period the 100% phosphoric acid [1245.6 g (12.71 moles)] fromPart (a) above, causing a temperature rise of 10° C. The solution ofphosphoric acid in isobutyl alcohol was cooled to 5°-10° C. To thiscooled solution was added, with stirring, 963.0 g (5.29 moles) ofvanadium pentoxide (V₂ O₅), followed by 1.35 g (0.032 mole) of lithiumchloride (LiCl), 0.96 g (0.017 mole or g-atom) of iron powder, and anadditional 1.0 1 of isobutyl alcohol, a portion of which was used torinse residual 100% phosphoric acid from its preparation vessel andtransfer funnel into the reaction flask. The charged P/V atom ratio wasabout 1.20, the Fe/V atom ratio was about 0.0016, the Li/V atom ratiowas about 0.0030, and the Fe/Li atom ratio was about 0.53. Anhydroushydrogen chloride [HCl (2037.0 g, 55.81 moles)] gas was added via thegas dispersion tube to the isobutyl alcohol/H₃ PO₄ /V₂ O₅ /LiCl/Femixture over a 4.67-hour period. During the HCl addition, thetemperature was maintained between 40° C. and 50° C. via a cooling bath.Upon completion of the HCl addition, at which time the initial yellowslurry had changed to a dark red-brown solution, the cooling bath wasremoved and replaced by a 12-1 heating mantle. The solution was heatedto reflux (initially 98° C.) over a 2.5-hour period, and maintained atreflux (approximately 102° C.) over an additional two-hour period.During the heat-up period and the reflux period, copious amounts of HClwere evolved and the solution changed transitorily from an initialred-brown to a green-brown to a navy blue color. Thereafter, 5.4 1 ofdistillate was removed at atmospheric pressure over a 5.0-hour period,followed by an additional 1.38-hour period of reflux, followed byremoval of an additional 1.5 1 of distillate over a 2.36-hour period,thereby removing a total of 6.9 1 of distillate over a 7.36-hourdistillate removal period. The turbid mixture was poured into two 4.445cm (1.75 in)×24.13 cm (9.5 in)×37.465 cm (14.75 in) Pyrex brand cakepans and placed in an oven maintained at 140°-150° C. for 5.5 hours toyield 2225.0 g of dry catalyst precursor. The dry catalyst precursor wasground and sieved to -14, +18 (14/18) mesh (U.S. Standard Sieve Size;1.0-1.4 mm) particles, placed in Pyrex brand casserole dishes, androasted by heating in a nitrogen/purged furnace to 260° C. over aone-hour period, which temperature was maintained for three hours,followed by a gradual replacement of the nitrogen by air and heating anadditional three hours to yield 1980.0 g of black catalyst precursorpowder. The dry precursor powder was mixed with one weight percent ofpowdered graphite (which serves as a tableting lubricant) and pressedinto 0.48-cm (0.1875-in) tablets having a side crush strength of22.25-44.50 newtons [N, 5.00-10.00 pounds (lbs)]. The catalyst precursortablets were calcined in situ in the presence of an atmosphere flowingat 100 hr⁻¹ total space velocity throughout the calcination period toconvert the catalyst precursor into the active catalyst. The tablet werecharged to a 2.12-cm (0.834-in.) inside diameter×335.28-cm (11-ft) longtubular fixed bed reactor maintained at 200° C. and heated in a flowingdry air atmosphere to 250° C. over a 3.124-hour period. Thereafter, thetemperature was allowed to drop slightly to 230° C., at which time waterwas added to the flowing dry air stream in an amount sufficient toprovide a water concentration of 1.8% by volume. The temperature wasincreased to 280° C. at a rate of 3° C. per hour and n-butane was addedto the flowing water-containing air stream in an amount sufficient toprovide an n-butane-in-air concentration of 0.6 mole percent. Thetemperature was increased to 400° C. at a rate of 1° C. per hour andthere maintained for a period of six hours, with the last five hoursbeing conducted under a flowing stream of nitrogen gas. The catalyst wasthen performance tested in the calcination reactor at 1150 hr⁻¹ spacevelocity and 1.9 mole percent n-butane-in-air. The parameters andresults are tabulated in Table 1.

EXAMPLE 2

The apparatus and procedure described in Example 1 was repeated exceptthat 9.5 g (0.035 mole) of ferric chloride hexahydrate was employed toprovide a charged Fe/V atom ratio of about 0.0033 and an Fe/Li atomratio of about 1.094. The resultant catalyst was performance tested asdescribed in Example 1. The parameters and results are tabulated inTable 1.

EXAMPLE 3

A catalyst was prepared using the apparatus and in accordance with theprocedure described in Example 1 except that 0.785 g (0.014 mole) ofiron powder was employed to provide a charged Fe/V atom ratio of about0.0013 and Fe/Li atom ratio of about 0.44. The catalyst was performancetested as described in Example 1 except that the space velocity was 2600hr⁻¹ and the n-butane-in-air concentration was 2.0 mole percent. Theparameters and results are tabulated in Table 1.

EXAMPLE 4 (Comparative)

The apparatus and procedure described in Example 1 was repeated exceptthat iron was omitted. The resultant catalyst was performance tested asdescribed in Example 1 at 1150 hr⁻¹ space velocity and 1.9 mole percentn-butane-in-air (4a) and at 2600 hr⁻¹ space velocity and 2.0 molepercent n-butane-in-air (4b). The parameters and results are tabulatedin Table 1.

                                      TABLE 1.sup.1                               __________________________________________________________________________                              SPACE                       SEL.                                       FORM   VELOCITY                                                                             n-BUTANE                                                                             TEMP. °C.                                                                       CONV.                                                                              mole                                                                              YIELD               EXAM.                                                                              EMPIRICAL FORMULA.sup.2                                                                     (SIZE), cm                                                                           hr.sup.-1                                                                            mole % BATH                                                                              REAC.                                                                              mole %                                                                              %  mole                __________________________________________________________________________                                                              %                   1    P.sub.1.20 V.sub.1.00 Fe.sub.0.0016 Li.sub.0.0030 O.sub.x                                   Tablets (0.48)                                                                       1150   1.9    413 451  78.1 69.8                                                                              54.5                2    P.sub.1.20 V.sub.1.00 Fe.sub.0.0033 Li.sub.0.0030 O.sub.x                                   "      1150   1.9    416 467  78.9 67.3                                                                              53.1                3    P.sub.1.20 V.sub.1.00 Fe.sub.0.0013 Li.sub.0.0030 O.sub.x                                   "      2600   2.0    427 482  71.4 67.9                                                                              48.5                 .sup. 4a.sup.3                                                                    P.sub.1.20 V.sub.1.00 Li.sub.0.0030 O.sub.x                                                 "      1150   1.9    430 467  78.8 64.9                                                                              51.1                  4b.sup.3                                                                         P.sub.1.20 V.sub.1.00 Li.sub.0.0030 O.sub.x                                                 "      2600   2.0    436 482  69.4 65.7                                                                              45.6                __________________________________________________________________________     .sup.1 The catalysts were performance tested in 2.12 cm (0.834 in) inside     diameter × 335.28 cm (11ft) long tubular fixed bed reactor.             .sup.2 Subscript x is a number taken to satisfy the average valence           requirements of the remaining elements present.                               .sup.3 Comparative example.                                              

Comparison of the performance of the catalysts of the instant inventionwith that of the comparative catalyst clearly demonstrates theadvantages of the instant catalysts. In each instance, when compared atthe same space velocity, the catalysts of the instant invention(Examples 1 and 2 at 1150 hr⁻¹ space velocity and Example 3 at 2600⁻¹space velocity) demonstrates higher values for the conversion ofn-butane and selectivity to, and yield of, maleic anhydride than thosedemonstrated by the comparative catalyst (Example 4a at 1150 hr⁻¹ spacevelocity and Example 4b at 2600 hr⁻¹ space velocity.

Thus, it is apparent that there has been provided in accordance with theinstant invention, catalysts that fully satisfy the objects andadvantages set forth hereinabove. While the invention has been describedwith respect to various specific examples and embodiments thereof, it isunderstood that the invention is not limited thereto and that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the invention.

What is claimed is:
 1. A catalyst for the production of maleic anhydrideby the partial oxidation of non-aromatic hydrocarbons which consistsessentially of phosphorus, vanadium and oxygen and a promoter componentcontaining each of iron and lithium, the catalyst having an(iron+lithium)/vanadium atom ratio from about 0.0025 to about 0.0080,with the proviso that the iron/vanadium atom ratio is from about 0.0010to about 0.0040 and the lithium/vanadium atom ratio is from about 0.0015to about 0.0040, and with the further priviso that the iron/lithium atomratio is from about 0.30 to about 1.30.
 2. The catalyst of claim 1wherein the catalyst has a phosphorus/vanadium atom ratio from about0.50 to about 2.00.
 3. The catalyst of claim 2 wherein the catalyst hasa phosphorus/vanadium atom ratio from about 0.95 to about 1.20.
 4. Thecatalyst of claim 1 wherein the catalyst has an iron/vanadium atom ratiofrom about 0.0015 to about 0.0035 and the lithium/vanadium atom ratio isfrom about 0.0025 to about 0.0035.
 5. A catalyst for the production ofmaleic anhydride by the partial oxidation of nonaromatic hydrocarbonswhich consists essentially of phosphorus, vanadium, and oxygen and apromoter component containing each of iron and lithium, wherein thephosphorus/vanadium/promoter component atom ratio is about1.20/1.00/0.0043-0.0063.